Controlling transmission in



Aug. 7, 1951 V c. N. NEBEL Filed Dec. 31, 1946 /2 27 2a 13 a i I K 1414b U.(I.F N0./ A 510. we. 5'2??? 29:15: i 0 I GEN. ATT. L

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" oes-- .4 .3 .2 .l RELATIVE AMPLITUDE OF REFLECTED WAVES //v l/EN TORC. N. NEBEL A NE) Patented Aug. 7, 1951 CONTROLLING TRANSMISSION INMICROWAVE SYSTEMS Charles N. Nebel, Dcnville, N. J., assignor to BellTelephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application December 31, 1946,Serial No. 719,356

4 Claims.

This invention relates to microwave signal transmission and morespecifically to a method of and apparatus for compensating for wavereflections in such transmission systems.

Many microwave signal transmission systems comprise at least one sectionof rectangular wave guide. This wave guide may extend effectively in arectilinear direction, and include one or more impedance irregularitiesdue, for example, to coupling misterminations or variations incross-section arising during manufacture. The misterminations may befrequently occasioned by the well-known choke or flange couplings. Thevariations in cross-section may also be due to the introduction of abend or a twist into a rectilinear wave guide or to the fabricatedcorners in curvilinear wave guide which consists of several rectilinearsections of wave guide. Such impedance irregularies tend to cause wavereflections whereby standing waves tend to be produced in thetransmission system to impair the efficiency thereof.

The present invention contemplates an arrangement for minimizingreflected waves in a microwave signal transmission system.

The main object of the present invention is to compensate for impedanceirregularities inherent in a rectangular wave guide.

Another object is to control wave reflections.

A directional coupler is known in microwave signal transmission systemsfor separating refiected waves from incident waves for the purpose oftheir measurement. This coupler is essentially of the type disclosed inthe copending application of W. W. Mumford, Serial No. 540,252, filedJune 14, 1944. Any alteration of the microwave system that tends toreduce the amplitude of reflected Waves indicates a smaller standingwave or a decrease in the reflection coefficient of the impedanceirregularities causing the reflected Waves in the system. It has beenfound that the amplitude change in the reflected waves becomesincreasingly small as the standing wave tends to approach its idealvalue 1.0. Thus, measurements of the amplitude of reflected wavesprovide highly sensitive indications of improvements in standing wavesbelow a certain limit, say, for example, the value of 1.25.

In accordance with the present invention, the amplitude of the reflectedwaves that are inherent in a microwave signal transmission system forthe above'noted reasons is utilized to determine the amount ofcorrection requiredto be introduced into such system in order to reducethe standing wave therein to at least a tolerable limit, say, forexample, to the value of 1:10 or therebelow so as to approach moreclosely the ideal value 1.0. This correction involves the introductioninto the system of one or more additional reflected waves whose combinedamplitude is substantially equal to that of the inherent reflected wavebut degrees out of phase therewith whereby the inherent and additionalreflected waves tend to cancel each other.

In a specific embodiment of the present invention, the foregoingcompensation for reflected waves inherent in a known microwave signaltransmission system is effected by producing of one or more preselecteddeformities in the inside surface of a rectangular wave guide beingtested in the system by means of a suitable device employed in apredetermined manner. These deformities, as one illustration, constituteprojections formed integral with the surface of the wave guide beingtested at the opposite ends thereof and extending interiorly of the waveguide, each projection being spaced from the impedance irregularity tobe compensated for and from each other in a sense related to theWavelength of the signal waves being transmitted in the wave guide beingtested. As a consequence of these projections additional reflected wavesare introduced into the microwave system with such phase and combinedamplitude that the additional waves tend to cancel the reflected wavesand thereby tend to reduce the standing wave to at least the tolerablevalue.

The deforming device comprises a familiar type of vise-like apparatushaving the usual two opposing jaws, one of which is movable and theother fixed. The movable jaw includes a smooth surface for engaging asmall portion of the outer surface on one side of the wave guide, andthe fixed jaw includes a pin-like projection for engaging a smallportion of the outer surface of the opposite side of the same wave guidealong the longitudinal axis thereof. As the movable jaw is graduallybrought closer toward the fixed jaw to a predetermined extent, thepin-like projection on the latter jaw forms a dent in the outer surfaceof the wave guide. This results in a projection formed integrally withthe inner surface of the wave guide being tested and extendinginteriorly thereof. One or more of these dents provided at properlyspaced points on the wave guide being tested cause additional reflectedwaves which tend to cancel the initial reflected waves or to reduce theamplitudes of the latter waves and thereby reduce the standing Wave inthe microwave signaling system as previously mentioned.

For determining the location and number of the dents, a measuringcircuit adapted for use with signal waves whose frequencies lie in theultra-high range is connected alternately to the reflected wave andincident wave outlet taps of the directional .coupler. Pursuant tocertain readings in the measuring circuit when so connected, the firstdent is located at one end of the wave guide being tested a distance ofwavelengths from the impedance irregularity being compensated for, andeach additional dent is located a distance of wavelengths from the nextprevious one, 11. being an odd integer and A the Wavelength of thesignal waves in the interior of the wave guide being tested. Thissection of wave guide is then reversed end for end in the microwavesystem, and provided at its other end with one or more dents in asimilar manner.

' A like procedure is followed for providing one or more dents in one orboth ends of a bent and/or a twisted wave guide, of a wave guide havingone or more fabricated corners, that is, curvilinear wave guide made upof -a plurality of rectilinear sections of wave guide, and in theauxiliary wave guides associated with the reflected and incident waveoutlet taps of the directional coupler mentioned previously.

A feature of this invention is that wave guide previously rejected onthe ground of causing standing waves lying above a tolerable limit, sayfor example, the value 1.10 mentioned hereinbefore, may now be renderedsatisfactory in the manner aforesaid. This tends to result in an.economy of material, time and labor.

The invention will be readily understood from the following detaileddescription taken together with the accompanying drawing, in which:

Fig. 1 illustrates a known microwave signal transmission system modifiedin accordance with a specific embodiment of the invention;

Figs. 1A, 1B and 10 show portions of a microwave Wave system which maybe individually substituted in Fig. 1 and modified in accordance with a.specific embodiment of the invention;

. Fig. 2 is a curve representing action obtainable in Figs. 1, 1A, 1Band 10;

Fig. 3 is a side elevational view of a device utilized for practicingthe specific embodiment of the invention used in Figs. 1 and 1A, 1B and10;

Fig. 4 is an enlarged fragmentary elevational view of Fig. 3; and

Fig. 5 is a cross-sectional view taken along the line {5-5 in Fig. 1.

Referring to Fig. 1, a known circuit for testing a section of tubularwave guide 14 for its standing wave ratio comprises in sequence a signalgenerator Ill, a suitable variable attenuator II, a suitable frequencymeter l2, a directional coupler l3, the section 14 of tubular waveguide, and an adjustable resistive-card load l6. The wave guide 14, forthe purpose of this illustration, maybe made of copper or aluminum, is

1%; inches by inch in cross-section, has a Wall thickness ofapproximately V inch, and will be assumed to include the impedanceirregularities which are to be identified later on and to be compensatedfor in a manner to be explained subsequently. To the directional-couplertap 11 for reflected waves are serially connected a section [8 oftubular wave guide and a crystal-com verter meter it! of known typeincluding the usual pickup probe. To the directional coupler were forincident waves'are s'e'rially'connected a variable attenuator 2i and afixed attenuator 22 of 20 decibels, both of which are of known design.The generator iil provides signal waves having a frequency Within therange at which hollow wave guides are useful in signal transmissionsystems. The electromagnetic waves being transmitted in the wave guide44 are of the dominant mode having electric lines normal to the widesides thereof.

The directional coupler 13 comprises two directional couplers, each ofwhich is essentially of the type disclosed in Fig. 7 of the copendingapplication of W. W. Mumford, supra; and as shown in Fig. 1, includes amain wave guide 25, an auxiliary wave guide 26 provided with a tap [7 atits left-hand end serving as an outlet for reflected power, and aconventional resistive card 2'! at its right-hand end;and an auxiliarywave guide 23 supplied with a tap 29 at its right-hand end serving as anoutletfor incident power, and

the conventional resistive card 21 at its left-hand end. It will beunderstood that the taps I! and 23 comprise waveguides formed withsuitable configurations for electrically connecting the auxiliary waveguides 26 and 28, respectively, to the apparatus associated therewith.Three openings 29 couple the main wave guide 25 to the auxiliary waveguide 25; and three openings 20a,

similar to the openings 29, couple the main wave guide 25 to theauxiliary Wave guide 28. These coupling openings provide substantiallyequal coupling losses for the respective auxiliary wave guides. t Y

In the operation of Fig. 1 to'obtain the SWR of the wave guide 14, themeter 19 is initially connected as shown in Fig. 1, andthe attenuator IIis adjusted to' provide a preselected reading, say for example, ten percent of the scale of meter 19. Next, the meter 19 is connected to theoutput terminal of the fixed attenuator 22, and then the attenuator 21is adjusted to repeat the ten per centreading. on the meter IS. The sumof the reading of the variable attenuator 2| plus the ZO-decibelattenuation of the fixed attenuatorZZ constitute the front-to-back orF/B ratio of the wave guide It. This. sum is then referred to the curvein Fig. 2 to obtain the value of the SWR of thewave guide 14. Assuming,for example, an SWR of 1.10 as a tolerable maximum limit for the unknownwave guide [4, then the F/B ratio of this wave guide should beapproximately 32.6 decibels; and the relative amplitude of the reflectedwave should be approximately 0.3 shown in Fig 2 for the purpose of thisillustration.

In accordance with a specific embodiment of the present invention shownin Figs. 1, 3, 4 and 5, the SWR of the waveguide I4 may be reduced in amanner that will now be explained from a value lying outside thetolerable limit of 1.10 to a value lying within this limit, oralternately to a value approaching more closely to the ideal value of1.10 than the above-identified assumed tolerable limit.

Referring to Fig. 3, a vise-like clamp 30 comprises a U-shaped member 3|provided with a fixed pedestal 32 and a screw 33 adjustable by means ofa handle 34 on its uppermost end. At the lowermost end of this screw ispositioned a right-angled members 31, 31.

a s-cadre member 35-1ncluding1a' fiatunderzsurface'fifi. i911 oppositevertical sides of member 35 is .a pairof The pedestal :32, as shown inFig. 4, comprises a cylindrical *element 4|i mounted vertically onthemembe1n3| and whose upper-most end 41 is formed with La, substantiallysemispherical configuration whose diameter is approximately at inch. Thehandle 34 serves to raise and'lower the member 351With reference to thefixed pedestal :32 in the wellknown manner.

The operation of Fig. 3 .in-iconnectionwith.Figs.

'1 and 2is as follows: Letit be assumed that the 3|] is positioned onthe left-hand end of the wave guide 4 in proximity of the coupling |4a:in the manner illustrated 'in Fig. .3. :In this connection it will beobserved that the two rightangled members 31, 31 :serve to dispose thepedestal end 4| substantially on the center or longitudinal axis of theunder-surface of the wave guide l4. Next, the handle 34 of the clamp 30is so actuated as to increase gradually the pressure of the pedestal end4| on the under-surface of the wave guide l4, \without permitting thepedestal end 4| to makea permanent dent in the under-surface of thelatter wave guide. Simultaneously, therewith, the reading on the meter9'is noted. A reading other than one decreasing below the 10 per centpointed out previously would tend to indicate that the pedestal end 4|is improperly placed relative to the impedance irregularity inthecoupling |4a.

Accordingly, the clamp 30 is shifted :along the longitudinal axis of thewave guide 14 until there is found for the pedestal end 4| a position atwhich the pressure of the pedestal end '41 under control of the handle34 :as previously :mentioned provides on the meter '19 a reading whichtends to decrease below the initial 1011381 .cent. This position of thepedestal end 4| would tend to establish afurther impedance irregularityin the unknown wave guide l4 whereby .a further reflected wave isintroduced into the system of Fig. 1. .Since the .further reflected wavetends :to reduce the reading on the meter l9, such further reflectedwave tends to be out of phase with the reflected wave caused by theimpedance irregularity in the coupling |4a. So long as the meter.reading'tends to decrease, this procedure of shifting and operating theclamp 30 is repeated until eventually the clamp 30 is located at a pointon the wave guide l4 at which the maximum sensitivity of the decreasingmeter reading is obtained. The further reflected wave produced at thispoint is substantially 180 degrees out of phase with the reflected wavecaused by the coupling |4a.

In other words, the further impedance irregularity is locatedsubstantially wavelengths from the impedance irregularity in thecoupling Ma, n being an odd integer and k the wavelength of the signalWaves in the wave guide 14. .It .is .known that the =0: two

impedance irregularities will -:substantial1y cancel each other whenthey Lare-spaced 4 wavelengths L-apart, provided their amplitudes aresubstantially equal. .At the point where the pedestal Lend 4| is locatedapproximately wavelengths from the impedance irregularity in thecoupling l4a, therhandle 34 of the clamp 30 .is actuated until thepedestal end is-driven to its maximum depth .into the under-surface .ofthe unknown waveguide .|4, whereby a circular dent 5.0 shownin Figs..1,-=3 and .5.is permanently .produced in under-surface or one wide sideof the unknown wave guide J4. This denthas-a depth of theorder .ofone-@sixteenth inchianda diameter of approximately inch .for the size ofthe waveguide I4 previously mentioned, and results in the production ofa tapering projection 50a, Fig. 5, formed integral with the innersurface of the lower wideside of the wave guide 14 and extendinginteriorly thereo'f. .In Fig. 5, the dent and projection are not shownto scale but .are somewhat enlarged .for the purpose of this:illustration; anditwill be obvious that the dimensions of the dent andprojection may vary .ior different sizes of waveguide.

Shouldit happen thatthe effect 'of'th'e one dent 50 'fails to cancel theeffect of the impedance irregularity in the coupling 14a, this meansthat "the amplitude 'of'the further-reflected wave'caused by the dent 50is'less than the amplitude of the reflected wave caused by the impedanceirregularity in the coupling Ma. Additional improvements in theeffective cancelationof the reflected wave due to the impedanceirregularity in the coupling |4a may be obtained by producing oneor-more additional dents 50 in the wave guide I 4, each dent "beingspaced wavelengths from adjacent dents. dent is located wavelengths fromthe impedance irregularity in the coupling |4a. .As a consequence, thefamplitudes of the several further reflected waves due to the individualdents 50 effectively add *vectorially to cancel substantially thereflected wave caused by the impedance irregularity inherent in thecoupling Ma.

The impedance irregularity in the flange 14b of the wave guide |4 may becompensated for by reversing the latter wave guide end for end inFig. 1. so that the couplings Ma and 14b are interchanged in so far astheir relative positions in Fig. l are concerned, and thereafterproducing one or more dents 50 in the end of the wave guide .|4 adjacentthe coupling I41) until the effect of the impedance irregularities dueto the dents 50 substantially cancel the effect of the impedanceirregularity inherent in the coupling |4b as here- .inbefore explained.

Over a range of samples of wave guide similar to the wave guide 14 inFig. 1, it was found that a maximum of four dents 5|] introduced asuilicient number of impedance irregularities at each end of the samplewave guides to compensate for the impedance irregularity inherent in thecoupling located at each end of the sample Wave guides; and further thatsuch number of dents 50 had an adequate factor of safety to prevent avoltage breakdown between the projections 59a, Fig. 5, and the internalsurface of the opposite side of the wave guide. In addition, the dent 50and projection 50a have the advantages of (a) being pressuretight wherethe wave guide M is operated undergas pressure, and (b) not affectingthe band-width characteristic of the wave uide 14. I

It has also been found that one or more dents 60, similar to dent 50,may be formed in the wave guide I4 at spaced points along thelongitudinal axis thereof to correct for one or more impedanceirregularities located elsewhere in the waveguide 14 but not in thecouplings thereof and due, for example, to variations in thecross-section thereof. In this connection, it will be understood thatone or more dents "60 may be located in the manner previously describedrelative to the production of dents 50 adjacent the ends of the waveguide [4.

An impedance irregularity in the coupling at each end of waveguidesections 52 and 53 shown in Figs. 1A and 13, respectively may becompensated for by substituting these individual sections to the rightof the line aa for the section M in Fig. 1, and thereafter producing oneor more permanent dents 50 and projections 50a in both ends of therespective sections, adjacent the couplings thereof in themannerpreviously. explained in connection with wave guide 14 in Fig. 1.Additional dents may be produced in the bends of the sections 52 and 53in Figs. 1A and 13 to compensate for impedance irregularities inherentin these bends, and may be located in the manner previously describedrelative to the production of dents 50 in the wave guide section 14.

It will be understood that the aforenoted procedure may be employed tocompensate for impedance irregularities in tubular wave guide sectionswhose initial SWR. is above the tolerable value 1.10 so as to reduce theSWR of these samples at least to this tolerable value, or alternately toa value therebelow, if possible. This would tend to reduce the quantityof wave guide rejected on the ground of having a SWIR. in excess of thetolerable limit of 1.10.

Figs. 1, 1C and 3 comprise an arrangement for compensating for impedanceirregularities inherent in the directional coupler l3 and due, forexample, to imperfect coupling openings 29 and 29a, the resistive cards27, and variations in cross-section. To accomplish this compensation,the apparatus included in the box 60a in Fig. 1C is substituted for theapparatus included in the box 6| in Fig. 1. Box 60a comprises thedirectional coupler I3 and the usual adjustable resistive-card load Hi.In the operation of Figs. 1, 1C and 3, the F/B ratio is measured byeffecting the per cent readings on the meter [9 in the manner explainedhereinbefore regarding the F/B ratio for the wave guide M in Fig. 1.This ratio for the directional coupler [3 in Fig. 1C should beapproximately 35 decibels for the average sample thereof before theapplication of the compensating dents 50 and projections 50a thereto.

Then, with the meter [9 connected to the tap I! in Figs. 1 and 1C, theclam 30 is disposed on the auxiliary wave guide 26 in Figs. 1 and 10 inproximity of the three coupling openings 29, in a manner similar to thatshown in Fig. 3, except one right-angled member 31 is removed from theclamp 30. As the pressure of the pedestal end 4| is graduallyincreasedon the under-surface of auxiliary wave guide 26 withoutproducing a permanent mark therein as pointed out previously concerningwave guide M, the reading of meter i9 is noted. As this reading tends todecrease, then the clamp 30 is shifted a small distance to the right orleft of the initial point and the clamp 30 operated again, So long asthe meter reading tends to decrease, this procedure of the shifting andoperating the clamp 30 is repeated until eventually the clamp 30 islocated on a point of the auxiliary wave guide 26 at which the maximumsensitivity of the decreasing meter reading is obtained. The furtherreflected wave produced at this point is substantially degreesout-of-phase with the unwanted reflected Wave in the directional couplerat the reflected-wave tap. Accordingly, the clamp 30 is operated toproduce a permanent dent 50 and projection 5041, Figs. 10 and 5, in theauxiliary wave guide 26 in the manner above described regarding unknownwave guide M. This dent, as above-mentioned, will be locatedapproximately wavelengths from the impedance irregularity to becorrected for. Additional dents, each spaced a distance wavelength fromthe next previous one, are produced in the auxiliary wave guide 26 asrequired, as mentioned hereinbefore-concerning unknown wave guide [4.The F/B is then measured by connecting the meter I9 to the directionaltap 20, and thereafter adjusting the attenuator 2i until the meterreading substantially equals the meter reading when the meter [9 waslastconnected to the tap H. The F/B ratio is the sum of .the reading ofattenuator 2| plus the fixed ZO-decibel attenuation of fixed attenuator22. The directional coupler I3 is then reversed end for end,

and the foregoing correction comprising one or more dents 50 andprojections 50a are produced in the auxiliary wave guide 28. Both tapsl1 and 20 are suitably corrected for the above-noted impedanceirregularities, when the F/B ratio is greater than 50 decibels for eachof these taps.

What is claimed is: 1. A tubular uniconductor wave guide for highfrequency electromagnetic Waves having a plurality of permanentindentations in the wall thereof spaced along the guide in suchcorrelation with the length of said waves in the guide as to have acumulative reactive effect on said waves, said indentations projectinginto the interior of said guide a distance small compared with thecrosssectional dimensions of the interior of said guide.

2. A tubular uniconductor wave guide for high frequency electromagneticwaves including an inherent impedance irregularity in said guide, meansfor exciting high frequency electromagnetic waves therein, and at leastone integral indentation in a wall of said guide in the form of arounded point-like protuberance extending inwardly of said guide, saidindentation being located substantially an integral multiple ofonequarter wavelengths of the mean frequency of said waves in said guideaway from said irregularity.

3. The structure according to claim 2 including a plurality of saidindentations spaced apart longitudinally along a wall of said guidesubstantially an integral mutiple of one-half wavelengths of the meanfrequency of said waves in said guide as to have a cumulative reactiveeffect on said waves.

4. A tubular wave guide for transmitting electromagnetic waves andincluding an inherent impedance irregularity causing wave reflection, animperforate dent formed in a wall of said guide, said dent forming anintegral point-like bulge extending inwardly of said guide of suchdimensions and having such location with reference to said impedanceirregularity in terms of the wavelength of the waves being transmittedin said guide as to introduce into said guide a further wave reflectionin such sense as tends m to cancel the first-mentioned wave reflection.CHARLES N. NEBEL.

10 REFERENCES CITED The following references are of record in the fileof this patent:

UNITED STATES PATENTS Number Name Date 2,108,640 Bieling Feb. 15, 19382,374,498 Quayle Apr. :24, 1945 2,400,777 Okress May 21, 1946 2,403,289Kormar July 2, 1946 2,407,068 Fiske et a1. Sept. 3, 1946 2,419,613Webber Apr. 29, 1947 2,423,383 Hershberger July 1, 1947 2,432,093 FoxDec. 9, 1947 2,433,368 Johnson et a1 Dec. 30, 1947 FOREIGN PATENTSNumber Country Date 503,467 Great Britain Apr. 6, 1939

